1. Field of the Invention
The present invention relates generally to a cathode-ray tube apparatus and more particularly to a color cathode-ray tube apparatus capable of improving an oval distortion of a beam spot, which is formed by an electron beam that is focused on a peripheral portion of a phosphor screen, and stably providing a high image quality with high resolution over the entire phosphor screen.
2. Description of the Related Art
In general, a self-convergence type in-line color cathode-ray tube apparatus comprises an in-line electron gun assembly for emitting three in-line electron beams, which travel on a horizontal plane. A dynamic focus type electron gun assembly, which is a type of in-line electron gun assembly, includes three in-line cathodes K, and first to fourth grids G1 to G4 that are successively arranged toward a phosphor screen, as shown in FIG. 17.
In this electron gun assembly, a voltage of about 190 V is applied to the cathodes K. The first grid G1 is grounded, and a voltage of about 800 V is applied to the second grid G2. A focus voltage of about 7 kV is applied to a first segment G3-1 of the third grid, and a reference voltage of about 7 kV is applied to a second segment G3-2 of the third grid. A high voltage of about 30 kV is applied to the fourth grid G4. An AC component, which varies in a parabolic fashion in accordance with a distance of deflection of an electron beam, is superimposed on the reference voltage that is applied to the second segment G3-2.
Thus, the cathodes K, first grid G1 and second grid G2 form an electron beam generating section that generates electron beams and forms an object point for the main lens. The second grid G2 and the third grid G3 form a prefocus lens that prefocuses the electron beams generated from the electron beam generating section. The third grid G3 and fourth grid G4 form a main lens that ultimately focuses the electron beams, which are prefocused by the prefocus lens, on a phosphor screen.
When the electron beams are deflected onto a corner area of the phosphor screen, the potential difference between the second segment G3-2 and the fourth grid G4 takes a minimum value and also the power of the main lens takes a minimum value. At the sane time, a quadrupole lens is created by a potential difference between the first segment G3-1 and second segment G3-2 of the third grid, and the lens power of the quadrupole lens takes a maximum value. The quadrupole lens is configured to have a focusing function in the horizontal direction and a diverging function in the vertical direction.
Thus, the movement of the focal point is compensated by decreasing the power of the main lens, as the distance between the electron gun assembly and phosphor screen increases and the image point moves farther. In addition, the quadrupole lens compensates a deflection aberration due to the deflection magnetic fields.
In recent years, with an increasing popularity of Hi-Vision broadcast and Internet TV, there is a demand that a color cathode-ray tube should display an image with higher definition. In order to display a high-definition image, it is desirable that a small beam spot with a substantially circular shape be formed over the entire phosphor screen.
In order to form a small beam spot, it is effective to decrease the magnification of the main lens. An example of a method for decreasing the magnification is to increase the aperture of the main lens. Jpn. Pat. Appln. KOKAI Publication No. 9-180648 discloses a method of increasing the aperture of the main lens, wherein a superimposition/extension type main lens is used.
As is shown in FIG. 18, the superimposition/extension type main lens includes an intermediate electrode GM that is disposed between the second segment G3-2 and fourth grid G4. The intermediate electrode GM has cylindrical electrodes at its portions opposed to the second segment G3-2 and fourth grid G4, respectively. The second segment G3-2 and fourth grid G4 hare electric field correction plates at their faces opposed to the intermediate electrode GM. Thereby, a superimposition/extension type main lens with a large aperture can be formed.
However, in the case where the focal distance of the lens system in the electron gun assembly is fixed, if the aperture of the main lens is increased, the power of the main lens needs to be increased in order to meet the focusing condition for just focusing electron beams on the phosphor screen. For this purpose, it is necessary to decrease the focus voltage, thereby increasing the potential difference from the anode voltage.
When the focus voltage is decreased, the potential difference between the second grid G2 and first segment G3-1 decreases and the lens power of the prefocus lens lowers. As a result, the divergence angle of the electron beam increases. When the electron beam, whose divergence angle is increased, is incident on the main lens, it is greatly affected by the spherical aberration of the main lens. Consequently, the site of the beam spot of the electron beam that reaches the phosphor screen increases.
When the focus voltage that is applied to the first segment G3-1 is decreased, the potential permeation to the second grid G2 decreases and the potential in the electron beam generating section decreases. Consequently, the size of the imaginary object point of the electron beam for the main lens increases. As a result, the beam spot of the electron beam that reaches the phosphor screen increases.
As has been described above, in the color cathode-ray tube apparatus, in order to display a high-definition, high-resolution image, it is necessary to form a small beam spot with no oval distortion on the entire phosphor screen.
As the method of decreasing the size of the beam spot on the phosphor screen, it is an effective method to increase the aperture of the main lens and decrease the magnification of the main lens. In this method, however, the lens power of the prefocus lens decreases, as mentioned above. Consequently, the divergence angel of the electron beam increases before it enters the main lens, and the electron beam is affected by the spherical aberration on the peripheral region of the main lens. At the same time, in this method, the potential permeation from the first segment to the second grid decreases, and the size of the imaginary object point increases. These phenomena lead to an increase in beam spot size, which is contradictory to the purpose of this method. Hence, it is not possible to form a sufficiently small beam spot on the phosphor screen.
According to a method wherein the focus electrode is elongated in the direction of travel of the electron beam, the focus voltage, which has decreased due to the increase in aperture of the main lens, can be raised. However, in the case where the focus voltage is raised by this method, the size of the electron beam, before entering the main lens, is increased and the electron beam is strongly affected by the spherical aberration of the main lens. As a result, a sufficiently small beam spot cannot be formed on the phosphor screen.